1. Field of the Invention
The present invention relates to surface-emitting lasers and methods for producing the lasers.
2. Description of the Related Art
A vertical-cavity surface-emitting laser (VCSEL) is a laser that emits laser light perpendicularly to the plane of its semiconductor substrate.
This type of surface-emitting laser generally includes a distributed Bragg reflector (DBR), typically formed by alternately stacking high-refractive-index layers and low-refractive-index layers with an optical thickness of λ/4.
Surface-emitting lasers have advantages over edge-emitting lasers in that they provide a stable single mode in terms of longitudinal-mode characteristics, have low thresholds, and can readily be produced as two-dimensional arrays. Surface-emitting lasers are expected to be applied to light sources for use in optical communication or transmission or in electrophotography.
An important challenge in surface-emitting lasers is to satisfactorily control the transverse mode of oscillation; a single transverse mode output is demanded for applications such as communication. To provide a single transverse mode, for example, a current confinement structure is formed in a surface-emitting laser by selective oxidation to limit the emission region of an active layer and a waveguide structure is simultaneously formed with the selectively oxidized portion.
In this method, the diameter of confinement must be reduced to achieve oscillation in a single transverse mode. A reduced diameter of confinement, however, results in a narrowed emission region, thus making it difficult to produce a high laser output.
Japanese Patent Laid-Open No. 2006-073823 (Patent Document 1), for example, proposes a surface-emitting laser that includes a multilayer reflector defining a resonator and having a higher reflectance in the center of the reflector and a lower reflectance in the periphery of the reflector. According to Patent Document 1, this surface-emitting laser can suppress oscillation in higher-order modes to achieve oscillation in a single transverse mode even for larger diameters of current confinement than normal after selective oxidation.
The surface-emitting laser disclosed in Patent Document 1 will now be described with reference to FIGS. 4A and 4B.
FIG. 4A shows a schematic structure of the surface-emitting laser as an example of the related art. The top of the laser has a mesa structure. A lower reflector 420, an active layer 430, a first upper reflector 440, and a second upper reflector 450 are sequentially formed on a substrate 410. An insulating film 460 and an electrode 470 are formed on the second upper reflector 450. The second upper reflector 450 includes an aluminum oxide region 452 formed by selective oxidation in the periphery thereof and an unoxidized region 451 in the center thereof.
FIG. 4B is a schematic enlarged view of the right half of the second upper reflector 450. The second upper reflector 450 is formed so that the aluminum content (x) of AlxGa1-xAs layers, functioning as low-refractive-index layers, increases gradually in a direction away from the active layer 430.
Next, a method for forming the first upper reflector 440 and the second upper reflector 450 will be described.
First, the high-refractive-index layers and low-refractive-index layers of the upper reflectors 440 and 450 are stacked on top of each other. The thickness t of each layer satisfies the condition t=λ/(4n), where λ is the laser oscillation wavelength and n is the refractive index of the individual layers.
The stacked films are then oxidized in the transverse direction by heat treatment in a steam atmosphere. The low-refractive-index layers of the second upper reflector 450 have such a composition that the aluminum content increases gradually in the direction away from the active layer 430. Because the oxidation rate increases with the aluminum content, the aluminum oxide region 452 is formed in the second upper reflector 450 such that the area of the region 452 increases in the direction away from the active layer 430, as shown in FIGS. 4A and 4B.
Because aluminum oxide has a lower refractive index than AlxGa1-xAs, namely, about 1.8, the condition satisfied before the oxidation, t=λ/(4n), is no longer satisfied after the oxidation. This results in a higher reflectance in the center of the second upper reflector 450 and a lower reflectance in the periphery of the reflector 450. According to Patent Document 1, the multilayer reflector with such a reflectance distribution suppresses oscillation in higher-order modes.
In the surface-emitting laser according to Patent Document 1, as described above, the aluminum-containing layers are selectively oxidized in the transverse direction to decrease the reflectance of the periphery of the layers.
In the transverse oxidation proposed in Patent Document 1, however, the oxidation rate increases rapidly as the oxidation approaches the center of the multilayer reflector. It is therefore difficult to terminate the oxidation reaction proceeding toward the center of the multilayer reflector in the transverse direction at a target position.